1. Field of the Invention
The present invention relates to a video projection device, and more specifically to a video projection device (for example, projector for digital cinema) having a converter lens attachably and detachably provided for a projection lens.
2. Description of Related Art
In a theater, to deal with various image face formats, a converter lens is attached to a projection lens. Converter lenses are not limited to those for projection (for example, for a projector), but those for photographing (for example, for a camera) are conventionally known. Moreover, they are divided into a plurality of types depending on differences in conversion magnification or attachment and detachment position. For example, as types having different conversion magnifications (wide type for enlarging the angle of view, a teletype for reducing the angle of view, etc.), a wide converter lens, a teleconverter lens, an anamorphic converter lens (for example, of an aspect ratio switching type), etc. are known. As types having different attachment and detachment positions, a front converter lens, a rear converter lens, and an internal converter lens, etc. are known.
Listed as examples of the front converter lens are: wide converter lenses suggested in Patent Documents 1 to 3; and teleconverter lenses suggested in Patent Documents 4 and 5. In addition, listed as examples of the anamorphic converter lens for CinemaScope switching are: an internal converter lens and a rear converter lens suggested in Patent Documents 6 and 7; and front converter lenses suggested in Patent Documents 8 and 9.    Patent Document 1: JP-A-2001-159733    Patent Document 2: JP-A-2005-62212    Patent Document 3: JP-A-2003-241094    Patent Document 4: JP-A-2006-84739    Patent Document 5: JP-A-2003-156685    Patent Document 6: JP-A-2000-249919    Patent Document 7: JP-A-2004-309895    Patent Document 8: U.S. Pat. No. 6,587,159    Patent Document 9: U.S. Pat. No. 5,339,193
When either of the internal converter lens and the rear converter lens (of the types described in Patent Documents 6 and 7, etc.) is used in a projection system, a large optical system is inserted in a portion where optical elements are held with high accuracy (that is, from a projection lens to a video display element). As a result, a mechanism required for maintaining high accuracy becomes large-scaled, increasing man-hour and costs. There also arise needs for holding the converter lens itself to the projection lens with high accuracy. Compared thereto, the front converter lens (of the type described in Patent Documents 1 to 5) can fulfill a converter function while maintaining high performance even with rough positional accuracy. Therefore, this requires only a simple attaching and detaching mechanism and easily performs switching.
The front converter lens is basically an afocal optical system. Thus, when the completely afocal converter lens is set to the projection lens with its focus adjusted to a screen located, for example, 50 m away, the focus position shifts out of the screen. Even when the converter lens itself has no focus mechanism, focus adjustment can be achieved with a focus mechanism of the projection lens when the converter lens is attached. However, the focus needs to be adjusted with the focus mechanism of the projection lens every time the converter lens is attached or detached, which complicates switching operation.
The front anamorphic converter lens suggested in Patent Document 9 is provided with a focus mechanism for correcting a difference between the vertical focus position and the horizontal focus position. However, this requires a plurality of cylindrical lenses, and complicates structure for linear driving in the focus mechanism, thus leading to cost increase. Moreover, configuration such that one of a plurality of types of projection lenses is replaceably loaded results in aberration deterioration depending on matching between the converter lens and the projection lens.